java中关于线程通信的锁和条件

Question

我是一名 java 初学者，我在学习 java 中的 Thread 时编写了以下代码。我认为，如果我锁定Resource.set() 并注释掉Lock.unlock()，则Resource.out() 中的代码无法执行，因为当我想执行out 方法时无法解锁。 BTW，不管我在set()还是out()注释掉解锁，程序都会这样执行：

Thread[Thread-1,5,main]....生产....chicken1
Thread[Thread-2,5,main]....消费..........chicken1
Thread[Thread-0,5,main]....生产....chicken2
Thread[Thread-3,5,main]....消费..........chicken2 ......

我想了很久，还是不明白。刚学，可能理解有误，望高人指教。 请原谅我糟糕的英语。非常感谢。我的代码在这里：

package Thread;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class ThreadStudying {

public static void main(String[] args) {
    Resource r = new Resource();
    Thread t0 = new Thread(new Producer(r));
    Thread t1 = new Thread(new Producer(r));
    Thread t2 = new Thread(new Consumer(r));
    Thread t3 = new Thread(new Consumer(r));

    t0.start();
    t1.start();
    t2.start();
    t3.start();
}

static class Resource {
    private String name;
    private int count = 1;
    boolean isOut = false;

    Lock lock = new ReentrantLock();
    Condition pro_con = lock.newCondition();
    Condition consu_con = lock.newCondition();

    public void set(String name) {
        lock.lock();
        try {
            while (isOut) {
                try {
                    pro_con.await();
                }
                catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

            this.name = name + count;
            System.out.println(Thread.currentThread() + "....Produce...." + this.name);
            count++;

            isOut = true;
            consu_con.signal();
        }
        finally {
            lock.unlock();
        }
    }

    public void out() {
        lock.lock();
        try {
            while (!isOut) {
                try {
                    consu_con.await();
                }
                catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

            System.out.println(Thread.currentThread() + "....Consume.........." + this.name);

            isOut = false;
            pro_con.signal();
        }
        finally {
            //lock.unlock();
        }
    }
}

static class Producer implements Runnable {
    Resource r;

    Producer(Resource r) {
        this.r = r;
    }

    public void run() {
        while (true) {
            r.set("chicken");

            try {
                Thread.sleep(500);
            }
            catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

static class Consumer implements Runnable {
    Resource r;

    Consumer(Resource r) {
        this.r = r;
    }


    @Override
    public void run() {
        while (true) {
            r.out();
            try {
                Thread.sleep(500);
            }
            catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
  }
}

Answer 1

在生产者和消费者中，您都在重复调用lock.await

while (true) {
    //
}

来自doc，当您拨打lock.await时：

与此条件关联的锁被原子释放

所以，无论你是否注释掉lock.unlock，生产者和消费者都不会被阻塞。

P.S. 使用以下代码记录有关获取和释放锁的更多详细信息：

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class ThreadStudying {

public static void main(String[] args) {
    Resource r = new Resource();
    Thread t0 = new Thread(new Producer(r), "Producer 1");
    Thread t1 = new Thread(new Producer(r), "Producer 2");
    Thread t2 = new Thread(new Consumer(r), "Consumer 1");
    Thread t3 = new Thread(new Consumer(r), "Consumer 2");

    t0.start();
    t1.start();
    t2.start();
    t3.start();
}

static class Resource {
    private String name;
    private int count = 1;
    boolean isOut = false;

    Lock lock = new ReentrantLock();
    Condition pro_con = lock.newCondition();
    Condition consu_con = lock.newCondition();

    public void set(String name) {
        System.out.println(Thread.currentThread() + "before lock");
        lock.lock();
        System.out.println(Thread.currentThread() + "get lock");
        try {
            while (isOut) {
                try {
                    System.out.println(Thread.currentThread() + "release lock");
                    pro_con.await();
                }
                catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

            this.name = name + count;
            System.out.println(Thread.currentThread() + "....Produce...." + this.name);
            count++;

            isOut = true;
            consu_con.signal();
        }
        finally {

        }
    }

    public void out() {
        System.out.println(Thread.currentThread() + "before lock");
        lock.lock();
        System.out.println(Thread.currentThread() + "get lock");
        try {
            while (!isOut) {
                try {
                    System.out.println(Thread.currentThread() + "release lock");
                    consu_con.await();
                }
                catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

            System.out.println(Thread.currentThread() + "....Consume.........." + this.name);

            isOut = false;
            pro_con.signal();
        }
        finally {
            //lock.unlock();
        }
    }
}

static class Producer implements Runnable {
    Resource r;

    Producer(Resource r) {
        this.r = r;
    }

    public void run() {
        while (true) {
            r.set("chicken");

            try {
                Thread.sleep(500);
            }
            catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

static class Consumer implements Runnable {
    Resource r;

    Consumer(Resource r) {
        this.r = r;
    }


    @Override
    public void run() {
        while (true) {
            r.out();
            try {
                Thread.sleep(500);
            }
            catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
  }
}

Answer 2

FirstOfAll，“如果我锁定 Resource.set() 并注释掉 Lock.unlock()，Resource.out() 中的代码将无法执行”。你的这种说法是错误的。

让我解释一下原因，

在您发布的代码中，out() 没有解锁。我假设您认为Consumer 线程之一（t2 或t3）在获取锁时没有问题。

假设t2 在进入out() 方法时获得了锁，而在退出out() 方法时没有释放锁。但是您忽略了out() 方法在run() 方法的Consumer Runnable 内无限循环执行的事实。所以当t2退出out()时，休眠500毫秒；它仍然拥有锁。当它在下一次迭代中进入out() 方法时，它会在它已经拥有的同一个锁上执行Lock.lock()。由于锁是ReentrantLock，它继续执行await()，在那里它释放锁；等待锁的其他线程（Producer线程）有机会获得锁。